- Email: [email protected]
High-power 1.5 and 3.4 μm intracavity KTA OPO driven by a diode-pumped Q-switched Nd:YAG laser
Optics Communications 282 (2009) 1668–1670
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
High-power 1.5 and 3.4 lm intracavity KTA OPO driven by a diode-pumped Q-switched Nd:YAG laser Xiao-Long Dong a, Bai-Tao Zhang a, Jing-Liang He a,*, Hai-Tao Huang a, Ke-Jian Yang a, Jin-Long Xu a, Chun-Hua Zuo a, Shuang Zhao a, Gang Qiu b, Zeng-Kai Liu b a b
State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Shanda South Road 27, Ji’nan 250100, China Crystech Inc, Qingdao 266100, China
a r t i c l e
i n f o
Article history: Received 8 October 2008 Received in revised form 24 December 2008 Accepted 24 December 2008
Keywords: Laser-diode-pumped Actively Q-switched Optical parametric oscillator
a b s t r a c t With a non-critically phase-matched KTA crystal, a high-power intracavity optical parametric oscillator (IOPO) driven by a diode-side-pumped acousto-optically Q-switched Nd:YAG laser has been realized. The maximum average output power of 13.6 W at the signal wavelength of 1534 nm and 3 W at the idler wavelength of 3472.7 nm were obtained with the repetition rate of 18 kHz, giving the optical–optical conversion efficiency of about 5.7% from diode-power at 808 nm to OPO signal output, which was the highest conversion efficiency for intracavity KTA OPO with diode-side-pumping configurations to our best knowledge. At the highest output power of 13.6 W, the signal pulse duration of 5.46 ns was obtained, corresponding to the single pulse energy of 756 lJ and peak power of 138 kW, respectively. Ó 2009 Elsevier B.V. All rights reserved.
High-power and high repetition rate eye-safe (1.5–1.6 lm) and mid-IR (3–5 lm) laser sources have wide applications in many fields, such as lidar systems, telemetry, range finders and remote sensing etc. The optical parametric oscillators (OPOs) pumped by the Q-switched neodymium lasers, supply a simple and efficient means to realize such high-power lasers [1]. Especially, the intracavity OPOs (IOPOs), which allow lower threshold and higher efficiency owing to the pump light’s multi-pass through the nonlinear crystal, make the configuration of the eye-safe laser much more compact and rugged. In recent years, the OPOs with KTP as the nonlinear crystal and pumped by the Q-switched Nd-doped lasers (such as Nd:YAG, Nd:YVO4, Nd:YAP, Nd:GdVO4 and Nd:KGW) have been extensively studied [2,3]. The most important advantage of the OPOs based on KTP and its isomorphs was that it can work under non-critical phase-matching (NCPM), which indicated that the KTP crystal allow a large acceptance angle. This made the KTP crystal permit efficient OPO operation even when pumped by multitransverse-mode lasers. However, the absorption at the idler wave (3–5 lm) in the crystal under NCPM conditions limits the augment of the output powers from KTP OPOs. KTiOAsO4 (KTA) crystal, as the isomorphs of KTP, has many merits similar to those of KTP, such as high transparency range (0.35–5.3 lm), high optical damage threshold, high nonlinear coefficient and low temperature sensitivity. Compared with KTP crystal, the KTA crystal used in the OPO systems could introduce lower intrinsic loss in the wavelength range of 3–4 lm, which * Corresponding author. E-mail address: [email protected] (J.-L. He). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.12.071
makes the KTA crystals mostly employed in high-power mid-infrared OPOs [4]. Until now, KTA crystals have been successfully used in IOPOs by several researchers [5,6]. However, mostly the signal average output power at 1.5 lm had not exceeded 5 W [7] and the idler average output power at 3.5 lm less than 2 W until that Wu et al. demonstrated an IOPO pumped by a diode-side-pumped Q-switched Nd:YALO laser that generated a OPO total average output power of 15 W under a diode pumping power of 570 W at 808 nm. However, the optical–optical conversion efficiency between the pump light at 808 nm and OPO total output was limited to be 2.6% [8]. In this paper, a high-power, high repetition rate, high conversion efficiency and high stability nanosecond IOPO emitting at the eye-safe wavelength and 3.5 lm has been realized. When a 25 mm-long KTA crystal was employed, the maximum average output power of 13.6 W at the signal wavelength of 1534 nm and 3 W at the idler wavelength of 3472.7 nm radiation were obtained with the repetition rate of 18 kHz, giving the optical–optical conversion efficiency of about 5.7% from diode-power at 808 nm to OPO signal output, which was the highest conversion efficiency for KTA IOPO by exploiting the laser module as the pump source to our best knowledge. At the highest signal output power of 13.6 W, the pulse duration at 1534 nm was 5.46 ns, corresponding to the peak power of 138 kW and the single pulse energy of 756 lJ. The experimental configuration of the intracavity KTA OPO was shown in Fig. 1. The length of the whole cavity was about 207 mm. The pump source was a Nd:YAG module, in which a 3-mm diameter and 65-mm-long water-cooled Nd:YAG rod was side-pumped by 12 diode arrays with an available maximum output power of
1669
X.-L. Dong et al. / Optics Communications 282 (2009) 1668–1670
M1
KTA
Nd:YAG
3472nm Idle Output Nd:YAG Pump Module
M2
M3
Fig. 1. The experimental configuration of the intracavity KTA OPO.
5 14 1.5µm 3.5µm
12
4
10 3 8 6
2
4 1 2 0
0
50
100
150
200
250
OPO Output Power at Idle Light (W)
approximately 240 W at 808 nm. M1 was a concave mirror with an 800 mm radius of curvature, high-reflection coated at 1064 nm. The A–O Q-switch (NEOS) was driven by r.f power of 50 W at a center frequency of 27 MHz. The A–O Q-switch was positioned as near as possible to mirror M1 for better hold-off at high gain of the 1064 nm laser. M2 was high reflectivity (R > 99%) coated at signal wavelength of 1534 nm and anti-reflectivity coated at the pump wavelength of 1064 nm. M3 was high reflectivity coated at 1064 nm and partially transmission at the signal wavelength. Three output mirrors with different transmission of T = 9.8%, 15.2% and 33.3% at the signal wavelength of 1534 nm are employed in our experiment. The mirror M1, the A–O Q-switch, the YAG module and mirror M3 formed the fundamental cavity, while the mirror M2, the KTA crystal and mirror M3 formed the OPO cavity. The whole OPO cavity length was 27 mm long. A type II noncritical phase-matching (x-axis, h = 90° and u = 0°) KTA crystal with the length of 25 mm was employed in our work, which was antireflection coated at 1534 nm and 1064 nm on both surfaces. To decrease the influence of the thermal effects, the KTA crystal was wrapped with indium foil and mounted in copper block cooled by water at the temperature of 21.7 °C. The laser pulse was recorded by a Tektronix DPO7104 digital oscilloscope (1 GHz bandwidth, 5 Gs/s sampling rate) and a photo-detector (New focus, model 1611). The performance of the diode-side-pumped Nd:YAG 1064 nm lasers in CW and Q-switching regimes were first studied. In order to get wider stability range for the fundamental oscillation at 1064 nm, the Nd:YAG module was placed in the middle of the whole cavity, that is, the distance from M1 was equal to that from the coupling output mirror. With the KTA crystal, M2 and the acousto-optical Q-switch moved from the cavity, the laser operated in CW regime. The maximum CW output power of 60 W at 1064 nm was obtained in a 207 mm long resonator with the output coupler mirror of T = 10%. When the Nd:YAG module worked in Qswitching regime, both mirror M1 and the acousto-optical Qswitch were positioned very close to the Nd:YAG module in order to shorten the whole cavity length and decrease the pulse duration. Under Q-switching regime, the maximum average output power of 45 W at 1064 nm was achieved at the repetition rate of 18 kHz, and the duration of the shortest pulse was 114 ns. Then with the KTA crystal and M2 inserted into the cavity and M3 used as the output mirror, the OPO operation was realized. When a thin-film polarizer was inserted into the laser cavity to obtain the linearly polarized 1064 nm radiation, the OPO output decreased at the same diode pump output. This should be attributed to the fact that the spatial profile of the elliptically polarized 1064 nm output from the Nd:YAG was changed to cross shaped owing to the birefringence in the Nd:YAG rod [9]. Therefore, we took the simple elliptically polarized setup. According to the OPO oscillation theory, there is an optimal relationship between the output coupler transmission of the OPO cavity and the repetition rate of the AO Q-switch for achieving the best output performance of the OPO. When the output coupler transmission was T = 15.2% at 1534 nm and the repetition rate was 18 kHz, the maximum output power of 13.6 W at 1534 nm and 3 W at
3472.7 nm were achieved under the pump power of 238 W, giving the optical–optical conversion efficiency of about 5.7% from diodepower at 808 nm to OPO signal output, which is the highest conversion efficiency for intracavity KTA OPO with diode-side-pumping configurations to our best knowledge. The higher conversion efficiency obtained in this IOPO could be attributed to two factors. The most important was the good pump beam quality, which was improved by choosing the Nd:YAG module with small rod diameter and adopting optimized cavity parameters. The other should be attributed to the relatively long KTA crystal. The average signal and idler output powers at 1534 nm and 3472.7 nm versus the pump power at 808 nm with the repetition rate of 18 kHz were shown in Fig. 2, respectively. Under the same absorbed pump power, the signal pulse width also slightly increased with the repetition rate increased, which should be attributed to the fact that there was not enough time for the gain medium to accumulate inversion populations enough at a higher repetition rate. However, the pulse widths at 1534 nm slightly decreased when increasing the absorbed pump power for the KTA crystal with the repetition rate fixed, which has been explained in the previous paper [10]. At the highest output power of 13.6 W, the signal pulse at 1534 nm was obtained with duration of 5.46 ns, corresponding to the single pulse energy and peak power of about 756 lJ and 138 kW. The corresponding signal waveform was shown in Fig. 3a, with the inset displaying the depleted pump radiation Fig. 3b. The depleted pump pulse has a relatively small pulse valley. This should be attributed to the fact that only the part of radiation which the electrical vector was situated in y-axis of KTA crystal during the IOPO operation has been transformed. It was also could be found that there was a strong shortening of the OPO pulses as compared to the pump pulses. This could be understood as follows. The duration of the signal pulse was of the order of the photon lifetime in the signal cavity, which was far smaller than the photon lifetime in the pump cavity. Once the intracavity intensity exceeded the threshold for parametric oscillation, the intracavity pump field was depleted in a single round-trip. The OPO oscillator was then quickly driven below the threshold, resulting in the generation of a short signal pulse. Furthermore, as described by the adiabatic theory by Debuisschert et al. [11], the lifetime of the photon in the pump cavity did not appear in the expressions of the FWHM of the signal pulse. Therefore, short signal pulses could be obtained even for long storage times of the pump photons. The M2 factors of the signal wave were measured to be 5.3 and 4.4 in the horizontal and vertical directions, respectively. Mean-
OPO Output Power at SignalLight (W)
AO Q-S
1534nm Signal Output
0
OPO Pump Power at 808nm (W) Fig. 2. The average signal and idler output powers at 1534 nm and 3472.7 nm versus the pump power at 808 nm with the repetition rate of 18 kHz.
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X.-L. Dong et al. / Optics Communications 282 (2009) 1668–1670
Fig. 3. The typical 1534 nm pulse shape with the pulse width of 5.45 ns and the inset corresponding to the depleted 1064 nm pulse shape.
while, it was worthwhile to mention that the red radiation at the wavelength of 628 nm with an output power of 13 mW was also obtained in our experiment. This should be attributed to the sum frequency mixing effect between the fundamental light at 1064 nm and signal light at 1534 nm, which requires a phasematching condition of (h = 90°, u = 24.8°) in the KTA crystal. In conclusion, a high-power, high repetition rate, high conversion efficiency and high stability eye-safe wavelength and 3.5 lm KTA OPO intracavity pumped by an actively Q-switched Nd:YAG with acousto-optic modulator has been demonstrated. Under the absorbed pump power of 238 W at 808 nm, a maximum average output power of 13.6 W at signal wavelength of 1534 nm and 3 W at idler wavelength of 3472.7 nm radiation were obtained simultaneously at the repetition rate of 18 kHz, the signal light pulse duration of 5.46 ns was achieved, corresponding to single pulse energy of 756 lJ and peak power of 138 kW, respectively. Acknowledgements The authors would like to thank Prof. X.Y. Zhang and Dr. Z.J. Liu for fruitful discussions. This work was supported by the Natural Science Foundation of China (Grant Nos: 10534020 and
50721002), the Program of Shandong Province Originality Innovation (Grant No: 2006GG1103047), and the Project for Shandong Taishan Scholars. The e-mail address of Jing-Liang He is [email protected]. References [1] Y.F. Chen, Opt. Lett. 29 (2004) 2172. [2] W. Zendzian, J.K. Jabczy´nski, P. Wachulak, J. Kwiatkowski, Appl. Phys. B 80 (2005) 329. [3] G.A. Rines, D.G. Rines, P.F. Moulton, in: T.Y. Fan, B.H.T. Chai (Eds.), Advanced Solid State Lasers of OSA Proceedings Series, Optical Society of America, Washington, DC, vol. 20, 1994, p. 461. [4] L.K. Cheng, L.-T. Cheng, J.D. Bierlein, F.C. Zumsteg, A.A. Ballman, Appl. Phys. Lett. 62 (1993) 346. [5] Xiaoyuan Peng, Lei Xu, Anand Asundi, IEEE J. Quant. Electron. 41 (2005) 53. [6] Larry R. Marshall, CLEO 1996, Techn. Dig. (1996) 368. [7] Zhaojun Liu, Qingpu Wang, Xingyu Zhang, Zejin Liu, Jun Chang, Hao Wang, Shuzhen Fan, Shutao Li, Shuaishuai Huang, Wenjia Sun, Guofan Jin, Xutang Tao, Shaojun Zhang, Huaijin Zhang, J. Phys. D: Appl. Phys. 41 (2008) 135112. [8] R.F. Wu, K.S. Lai, H. Wong, W.-J. Xie, Y. Lim, E. Lau, Opt. Exp. 8 (2001) 694. [9] Poh Boon Phua, Kin Seng Lai, Ruifen Wu, Appl. Opt. 39 (2000) 1435. [10] H.T. Huang, J.L. He, X.L. Dong, C.H. Zuo, B.T. Zhang, G. Qiu, Z.K. Liu, Appl. Phys. B 90 (2008) 43. [11] T. Debuisschert, J. Raffy, J.-P. Pocholle, M. Papuchon, J. Opt. Soc. Am. B 13 (1996) 1569.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
High-power 1.5 and 3.4 lm intracavity KTA OPO driven by a diode-pumped Q-switched Nd:YAG laser Xiao-Long Dong a, Bai-Tao Zhang a, Jing-Liang He a,*, Hai-Tao Huang a, Ke-Jian Yang a, Jin-Long Xu a, Chun-Hua Zuo a, Shuang Zhao a, Gang Qiu b, Zeng-Kai Liu b a b
State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Shanda South Road 27, Ji’nan 250100, China Crystech Inc, Qingdao 266100, China
a r t i c l e
i n f o
Article history: Received 8 October 2008 Received in revised form 24 December 2008 Accepted 24 December 2008
Keywords: Laser-diode-pumped Actively Q-switched Optical parametric oscillator
a b s t r a c t With a non-critically phase-matched KTA crystal, a high-power intracavity optical parametric oscillator (IOPO) driven by a diode-side-pumped acousto-optically Q-switched Nd:YAG laser has been realized. The maximum average output power of 13.6 W at the signal wavelength of 1534 nm and 3 W at the idler wavelength of 3472.7 nm were obtained with the repetition rate of 18 kHz, giving the optical–optical conversion efficiency of about 5.7% from diode-power at 808 nm to OPO signal output, which was the highest conversion efficiency for intracavity KTA OPO with diode-side-pumping configurations to our best knowledge. At the highest output power of 13.6 W, the signal pulse duration of 5.46 ns was obtained, corresponding to the single pulse energy of 756 lJ and peak power of 138 kW, respectively. Ó 2009 Elsevier B.V. All rights reserved.
High-power and high repetition rate eye-safe (1.5–1.6 lm) and mid-IR (3–5 lm) laser sources have wide applications in many fields, such as lidar systems, telemetry, range finders and remote sensing etc. The optical parametric oscillators (OPOs) pumped by the Q-switched neodymium lasers, supply a simple and efficient means to realize such high-power lasers [1]. Especially, the intracavity OPOs (IOPOs), which allow lower threshold and higher efficiency owing to the pump light’s multi-pass through the nonlinear crystal, make the configuration of the eye-safe laser much more compact and rugged. In recent years, the OPOs with KTP as the nonlinear crystal and pumped by the Q-switched Nd-doped lasers (such as Nd:YAG, Nd:YVO4, Nd:YAP, Nd:GdVO4 and Nd:KGW) have been extensively studied [2,3]. The most important advantage of the OPOs based on KTP and its isomorphs was that it can work under non-critical phase-matching (NCPM), which indicated that the KTP crystal allow a large acceptance angle. This made the KTP crystal permit efficient OPO operation even when pumped by multitransverse-mode lasers. However, the absorption at the idler wave (3–5 lm) in the crystal under NCPM conditions limits the augment of the output powers from KTP OPOs. KTiOAsO4 (KTA) crystal, as the isomorphs of KTP, has many merits similar to those of KTP, such as high transparency range (0.35–5.3 lm), high optical damage threshold, high nonlinear coefficient and low temperature sensitivity. Compared with KTP crystal, the KTA crystal used in the OPO systems could introduce lower intrinsic loss in the wavelength range of 3–4 lm, which * Corresponding author. E-mail address: [email protected] (J.-L. He). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.12.071
makes the KTA crystals mostly employed in high-power mid-infrared OPOs [4]. Until now, KTA crystals have been successfully used in IOPOs by several researchers [5,6]. However, mostly the signal average output power at 1.5 lm had not exceeded 5 W [7] and the idler average output power at 3.5 lm less than 2 W until that Wu et al. demonstrated an IOPO pumped by a diode-side-pumped Q-switched Nd:YALO laser that generated a OPO total average output power of 15 W under a diode pumping power of 570 W at 808 nm. However, the optical–optical conversion efficiency between the pump light at 808 nm and OPO total output was limited to be 2.6% [8]. In this paper, a high-power, high repetition rate, high conversion efficiency and high stability nanosecond IOPO emitting at the eye-safe wavelength and 3.5 lm has been realized. When a 25 mm-long KTA crystal was employed, the maximum average output power of 13.6 W at the signal wavelength of 1534 nm and 3 W at the idler wavelength of 3472.7 nm radiation were obtained with the repetition rate of 18 kHz, giving the optical–optical conversion efficiency of about 5.7% from diode-power at 808 nm to OPO signal output, which was the highest conversion efficiency for KTA IOPO by exploiting the laser module as the pump source to our best knowledge. At the highest signal output power of 13.6 W, the pulse duration at 1534 nm was 5.46 ns, corresponding to the peak power of 138 kW and the single pulse energy of 756 lJ. The experimental configuration of the intracavity KTA OPO was shown in Fig. 1. The length of the whole cavity was about 207 mm. The pump source was a Nd:YAG module, in which a 3-mm diameter and 65-mm-long water-cooled Nd:YAG rod was side-pumped by 12 diode arrays with an available maximum output power of
1669
X.-L. Dong et al. / Optics Communications 282 (2009) 1668–1670
M1
KTA
Nd:YAG
3472nm Idle Output Nd:YAG Pump Module
M2
M3
Fig. 1. The experimental configuration of the intracavity KTA OPO.
5 14 1.5µm 3.5µm
12
4
10 3 8 6
2
4 1 2 0
0
50
100
150
200
250
OPO Output Power at Idle Light (W)
approximately 240 W at 808 nm. M1 was a concave mirror with an 800 mm radius of curvature, high-reflection coated at 1064 nm. The A–O Q-switch (NEOS) was driven by r.f power of 50 W at a center frequency of 27 MHz. The A–O Q-switch was positioned as near as possible to mirror M1 for better hold-off at high gain of the 1064 nm laser. M2 was high reflectivity (R > 99%) coated at signal wavelength of 1534 nm and anti-reflectivity coated at the pump wavelength of 1064 nm. M3 was high reflectivity coated at 1064 nm and partially transmission at the signal wavelength. Three output mirrors with different transmission of T = 9.8%, 15.2% and 33.3% at the signal wavelength of 1534 nm are employed in our experiment. The mirror M1, the A–O Q-switch, the YAG module and mirror M3 formed the fundamental cavity, while the mirror M2, the KTA crystal and mirror M3 formed the OPO cavity. The whole OPO cavity length was 27 mm long. A type II noncritical phase-matching (x-axis, h = 90° and u = 0°) KTA crystal with the length of 25 mm was employed in our work, which was antireflection coated at 1534 nm and 1064 nm on both surfaces. To decrease the influence of the thermal effects, the KTA crystal was wrapped with indium foil and mounted in copper block cooled by water at the temperature of 21.7 °C. The laser pulse was recorded by a Tektronix DPO7104 digital oscilloscope (1 GHz bandwidth, 5 Gs/s sampling rate) and a photo-detector (New focus, model 1611). The performance of the diode-side-pumped Nd:YAG 1064 nm lasers in CW and Q-switching regimes were first studied. In order to get wider stability range for the fundamental oscillation at 1064 nm, the Nd:YAG module was placed in the middle of the whole cavity, that is, the distance from M1 was equal to that from the coupling output mirror. With the KTA crystal, M2 and the acousto-optical Q-switch moved from the cavity, the laser operated in CW regime. The maximum CW output power of 60 W at 1064 nm was obtained in a 207 mm long resonator with the output coupler mirror of T = 10%. When the Nd:YAG module worked in Qswitching regime, both mirror M1 and the acousto-optical Qswitch were positioned very close to the Nd:YAG module in order to shorten the whole cavity length and decrease the pulse duration. Under Q-switching regime, the maximum average output power of 45 W at 1064 nm was achieved at the repetition rate of 18 kHz, and the duration of the shortest pulse was 114 ns. Then with the KTA crystal and M2 inserted into the cavity and M3 used as the output mirror, the OPO operation was realized. When a thin-film polarizer was inserted into the laser cavity to obtain the linearly polarized 1064 nm radiation, the OPO output decreased at the same diode pump output. This should be attributed to the fact that the spatial profile of the elliptically polarized 1064 nm output from the Nd:YAG was changed to cross shaped owing to the birefringence in the Nd:YAG rod [9]. Therefore, we took the simple elliptically polarized setup. According to the OPO oscillation theory, there is an optimal relationship between the output coupler transmission of the OPO cavity and the repetition rate of the AO Q-switch for achieving the best output performance of the OPO. When the output coupler transmission was T = 15.2% at 1534 nm and the repetition rate was 18 kHz, the maximum output power of 13.6 W at 1534 nm and 3 W at
3472.7 nm were achieved under the pump power of 238 W, giving the optical–optical conversion efficiency of about 5.7% from diodepower at 808 nm to OPO signal output, which is the highest conversion efficiency for intracavity KTA OPO with diode-side-pumping configurations to our best knowledge. The higher conversion efficiency obtained in this IOPO could be attributed to two factors. The most important was the good pump beam quality, which was improved by choosing the Nd:YAG module with small rod diameter and adopting optimized cavity parameters. The other should be attributed to the relatively long KTA crystal. The average signal and idler output powers at 1534 nm and 3472.7 nm versus the pump power at 808 nm with the repetition rate of 18 kHz were shown in Fig. 2, respectively. Under the same absorbed pump power, the signal pulse width also slightly increased with the repetition rate increased, which should be attributed to the fact that there was not enough time for the gain medium to accumulate inversion populations enough at a higher repetition rate. However, the pulse widths at 1534 nm slightly decreased when increasing the absorbed pump power for the KTA crystal with the repetition rate fixed, which has been explained in the previous paper [10]. At the highest output power of 13.6 W, the signal pulse at 1534 nm was obtained with duration of 5.46 ns, corresponding to the single pulse energy and peak power of about 756 lJ and 138 kW. The corresponding signal waveform was shown in Fig. 3a, with the inset displaying the depleted pump radiation Fig. 3b. The depleted pump pulse has a relatively small pulse valley. This should be attributed to the fact that only the part of radiation which the electrical vector was situated in y-axis of KTA crystal during the IOPO operation has been transformed. It was also could be found that there was a strong shortening of the OPO pulses as compared to the pump pulses. This could be understood as follows. The duration of the signal pulse was of the order of the photon lifetime in the signal cavity, which was far smaller than the photon lifetime in the pump cavity. Once the intracavity intensity exceeded the threshold for parametric oscillation, the intracavity pump field was depleted in a single round-trip. The OPO oscillator was then quickly driven below the threshold, resulting in the generation of a short signal pulse. Furthermore, as described by the adiabatic theory by Debuisschert et al. [11], the lifetime of the photon in the pump cavity did not appear in the expressions of the FWHM of the signal pulse. Therefore, short signal pulses could be obtained even for long storage times of the pump photons. The M2 factors of the signal wave were measured to be 5.3 and 4.4 in the horizontal and vertical directions, respectively. Mean-
OPO Output Power at SignalLight (W)
AO Q-S
1534nm Signal Output
0
OPO Pump Power at 808nm (W) Fig. 2. The average signal and idler output powers at 1534 nm and 3472.7 nm versus the pump power at 808 nm with the repetition rate of 18 kHz.
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X.-L. Dong et al. / Optics Communications 282 (2009) 1668–1670
Fig. 3. The typical 1534 nm pulse shape with the pulse width of 5.45 ns and the inset corresponding to the depleted 1064 nm pulse shape.
while, it was worthwhile to mention that the red radiation at the wavelength of 628 nm with an output power of 13 mW was also obtained in our experiment. This should be attributed to the sum frequency mixing effect between the fundamental light at 1064 nm and signal light at 1534 nm, which requires a phasematching condition of (h = 90°, u = 24.8°) in the KTA crystal. In conclusion, a high-power, high repetition rate, high conversion efficiency and high stability eye-safe wavelength and 3.5 lm KTA OPO intracavity pumped by an actively Q-switched Nd:YAG with acousto-optic modulator has been demonstrated. Under the absorbed pump power of 238 W at 808 nm, a maximum average output power of 13.6 W at signal wavelength of 1534 nm and 3 W at idler wavelength of 3472.7 nm radiation were obtained simultaneously at the repetition rate of 18 kHz, the signal light pulse duration of 5.46 ns was achieved, corresponding to single pulse energy of 756 lJ and peak power of 138 kW, respectively. Acknowledgements The authors would like to thank Prof. X.Y. Zhang and Dr. Z.J. Liu for fruitful discussions. This work was supported by the Natural Science Foundation of China (Grant Nos: 10534020 and
50721002), the Program of Shandong Province Originality Innovation (Grant No: 2006GG1103047), and the Project for Shandong Taishan Scholars. The e-mail address of Jing-Liang He is [email protected]. References [1] Y.F. Chen, Opt. Lett. 29 (2004) 2172. [2] W. Zendzian, J.K. Jabczy´nski, P. Wachulak, J. Kwiatkowski, Appl. Phys. B 80 (2005) 329. [3] G.A. Rines, D.G. Rines, P.F. Moulton, in: T.Y. Fan, B.H.T. Chai (Eds.), Advanced Solid State Lasers of OSA Proceedings Series, Optical Society of America, Washington, DC, vol. 20, 1994, p. 461. [4] L.K. Cheng, L.-T. Cheng, J.D. Bierlein, F.C. Zumsteg, A.A. Ballman, Appl. Phys. Lett. 62 (1993) 346. [5] Xiaoyuan Peng, Lei Xu, Anand Asundi, IEEE J. Quant. Electron. 41 (2005) 53. [6] Larry R. Marshall, CLEO 1996, Techn. Dig. (1996) 368. [7] Zhaojun Liu, Qingpu Wang, Xingyu Zhang, Zejin Liu, Jun Chang, Hao Wang, Shuzhen Fan, Shutao Li, Shuaishuai Huang, Wenjia Sun, Guofan Jin, Xutang Tao, Shaojun Zhang, Huaijin Zhang, J. Phys. D: Appl. Phys. 41 (2008) 135112. [8] R.F. Wu, K.S. Lai, H. Wong, W.-J. Xie, Y. Lim, E. Lau, Opt. Exp. 8 (2001) 694. [9] Poh Boon Phua, Kin Seng Lai, Ruifen Wu, Appl. Opt. 39 (2000) 1435. [10] H.T. Huang, J.L. He, X.L. Dong, C.H. Zuo, B.T. Zhang, G. Qiu, Z.K. Liu, Appl. Phys. B 90 (2008) 43. [11] T. Debuisschert, J. Raffy, J.-P. Pocholle, M. Papuchon, J. Opt. Soc. Am. B 13 (1996) 1569.